Tunable microwave cavity using a piezoelectric device



TUNABLE MICROWAVE CAVITY USING APIEZOELECTRIC DEVICE lFiled May 5, 1967v Y A TTORIVEY WMMMM United States Patent O U.S. Cl. 333-83 3 Claims ABSTRACT F THE DISCLOSURE A piezoelectric device is affixed to a movable wall of a microwave cavity. Expansion and contraction of the piezoelectric material as a function of applied voltages is utilized to position the movable wall; and hence, tune the microwave cavity.

A microwave cavity; a cavity including conductive walls, possesses electrical properties similar to the conventional tuned circuit consisting of an inductance and a capacitance at low radio frequencies. At very high radio frequencies or microwave frequencies, a conductively walled enclosure or cavity possesses the familiar characteristics of resonance and impedance. With any given cavity, the frequency at which resonance occurs is primarily a function of cavity size or volume. Hence, as is known, a microwave tuner may be constructed with a cavity utilizing a movable wall therewith that is adjustably positionable relative to the other cavity walls so as to increase or decrease the elfective size of the cavity, thereby lowering or raising the resonant frequency of the cavity.

Various types of mechanical devices are provided in the prior art for positioning the movable cavity wall, including rods and screws, and which may be driven by mechanical, hydraulic or electromechanical arrangements.

There are limitations with these devices. A mechanically actuated movement for mechanically moving a cavity wall is relatively slow. Tuning at a relatively rapid rate is not possible due to mechanical inertia.

Other devices are found in the prior art which adjust or change the effective resonance characteristics of a microwave cavity other than by the mechanical movement of one of the cavity wa-lls. As an example, tuners are constructed utilizing conductive or dielectric rods or strips xedly placed within the cavity or adjustably positioned within the cavity by a screw or slot and screw arrangement. Additionally, ferroelectric devices are utilized as a tuner for a microwave cavity. In that instance, with a ferroelectric tuning device exposed to microwaves within a microwave cavity, the effective capacitance of the included ferroelectric; hence, the impedance of the microwave cavity, is adjustable with variations in the level of the voltage applied to the ferroelectric. In like manner, ferromagnetic or garnets exposed to microwaves are utilized as tuners for microwave cavities.

Electron discharge devices operable in the microwave frequency region, such as the klystron and magnetron, contain tunable cavities for adjusting the output frequency of the device. Such tunable cavities are used for setting the electron discharge device to the desired frequency and to modulate the frequency of the electron discharge device for jamming radar stations, in addition to other more exotic applications of a modulated or swept signal. Additionally, tunable cavities are used as adjustable or variable impedances.

Although the ferroelectric material possesses theoretical capabilities for purely electronic tuning without mov- ICC ing parts at relatively rapid rates, there are problems inherent in a relatively lossy ceramic ferroelectric material which render this alternative impractical in microwave cavities sustaining large magnitudes of microwave power. For example, at any signilicant power, the microwaves appearing within a microwave cavity heat a relatively lossy ceramic, such as the ferroelectric material, exposed to the elds within the cavity, causing it to decompose.

Therefore, it is an object of the invention to provide a rapid tuner which is protected against the heating effect caused by direct exposure to microwave energy.

It is a further object of the invention to provide a microwave tuner with no mechanical drive.

In accordance with the invention, a microwave cavity is provided having a plurality of conductive walls including a movable wall. The movable wall is connected to a piezoelectric device that possesses the property of varying its dimensions as a function of applied electrical voltages. Accordingly, the movable wall is positioned and the cavity is tuned by the movement which occurs due to expansion and contraction of the piezoelectric as a function of the voltage applied to the latter.

The foregoing and `other objects and advantages of the present invention become apparent from a reading of the specification in view of the illustrated figure, which shows a microwave cavity in accordance with the present invention.

The figure shows a microwave cavity 1 in section consisting of a plurality of conductively walled sides 2, 3, 4, 5, and 12. The cavity contains a passage 7 for permitting the exit or entrance of microwave energy and other passages may be provided. A support wall 6 is connected with the positionally iixed side walls 2, 3, and 5, and the unillustrated front wall to form a closed chamber containing the cavity 1. Disposed within the formed chamber is an elongated piece of piezoelectric material 8, rectangular in cross section.

Along two opposed sides of piezoelectric material 8 is a first conductive layer 10 and a second conductive layer 12, each of conductive material which forms, respectively, first and second electrodes. Conductive layer 12 forms one wall or side of the cavity in addition to its function as an electrode for the piezoelectric material. The piezoelectric material and electrodes are supported within the cavity 1 upon a layer of insulating material 9 and the bottom support wall 6. The insulating layer 9 can be deleted if electrical isolation is not desired. A rst electrical lead 13 is connected through wall 6, an insulator 14, through an opening in insulating layer 9, to electrode 10 in order to provide an electrical path for voltages from an electrical source, not illustrated. A second electrical lead 15 is connected through wall 3 and through an insulator 16 to the second electrode 12 in order to provide a second conductive path for electrical voltages from an external source, not illustrated.

As is known, piezoelectric ymaterial is inuenced by the application of an electrical field across portions thereof. By applying an electrical voltage between the two electrodes, the piezoelectric material very rapidly expands or contracts in an amount proportional to the magnitude and polarity of the applied voltages. The direction of expansion and/or contraction is dependent on the direction of polarization Iand cut of the piezoelectric material. In the disclosed embodiment, the application of voltages from a source across electrical leads 13 and 15; hence, across electrodes 10 `and 12 provides expansion and contraction of piezoelectric material 8 in a vertical `directionin dependence upon the magnitude of the applied voltage at any instant. The conductive layer 12 is thus positioned at various distances from upper wall 4 by such movement and Ivaries rthe sizeof the microwave cavity.

The source of voltages applied between electrical leads 13 `and 15 may be a periodically varying sweep voltage source, if periodic movement of the conductive layer 12 is desired. Alternatively, the voltage source may be an adjustable DC source which is utilized to position conductive llayer 12 at a Idesired location and maintain such position.

As is known, the frequency of resonance of a microwave cavity is predominantly la function of the size of the cavity. A movable cavity wall which borders a volume otherwise enclosed by other conductive walls changes the `frequency of resonance of the cavity as a function of the position of the movable wall. Inasmuch as the conductive layer 12 subsequentially extends about the entire bottom portion of the chamber and tfaces the upper wall 4, the conductive layer 12 effectively acts as the bottom wall of the volume vforming microwave cavity 1. The clearances between the ends of wall 12 and walls perpendicular thereto, such as 3 and 5 in the figure, is small relative to the wavelength of the microwave energy in the range of frequencies for which the microwave cavity is used. Conquently, such small clearances appear `as very high impedances to microwave energy within the cavity, and any leakage therein is negligible insofar as it affects the frequency of resonance of microwave cavity 1. Thus, under control of the voltages applied from an external source, rthe conductive layer 12 is very rapidly positioned to increase or decrease the size of the microwave cavity; and hence, to lower or raise the `frequency at which the cavity is resonant.

Each of the walls and 12 may be a metallic film deposited upon the piezoelectric material in any conventional manner, or alternatively, physically separate thin layers coupled mechanically to the piezoelectric layer at one or more locations.

The piezoelectric material 8 may be natural material or of the well known substances, such as Barium Titanate, which requires initially the application of a polarizing current to polarize the material.

It is noted that since the conductive wall or layer 12 substantially covers the piezoelectric material 8; hence, the piezoelectric material is effectively shielded or isolated from microwave energy 'appearing within cavity 1. This effectively prevents the microwave energy from heating the piezoelectric material.

While the microwave cavity of `FIGURE 1 has been described as rectangular in cross-section, it is apparent that the cavity may be constructed in any shape including the common circular cross-section cylindrical cavities without detracting from the invention. For instance, by deleting reference to one of Ithe side walls and back walls, the remaining wall 10 may be considered cylindrical in shape and FIGURE 1 may be considered a cross-section of a cylindrical cavity. Likewise, similar preferencesl in the selection in the shape of the piezoelectric material and its electrodes are allowable and within the scope of the disclosed invention.

The -fforegoing description land vdrawings are presented solely for purposes of illustration as many equivalents are apparent to one skilled in the art, :and are not intended to limit the invention as defined by the breadth and scope of the appended claims.

What is claimed is:

1. A microwave tuner comprising 'a microwave cavity having a plurality of conductive walls including: a movable wall; piezoelectric means connected to said movable wall; lirst and second electrodes connected to said piezoelectric means; means for lapplying voltages across said iirst and second electrodes comprising tirst and second |leads; microwave passage means connected to said cavity for permitting passage of microwave energy; a support wall connected to ya plurality of said other walls; a layer of insulating material substantially covering `the inner side of said support wall and said piezoelectric means being 'disposed on said layer of insulating material; whereby said piezoelectric means varies its dimensions in response 'to the magnitude of applied voltages to position the movable wall and said cavity is tuned in |dependence thereon.

2. The invention as defined in claim 1 wherein said support wall and said plurality of fixed walls form a chamber.

3. The invention as defined in claim 1 wherein said movable wall and said lirs-t electrode means comprise an integral conductive layer upon said piezoelectric means.

References Cited UNITED STATES PATENTS 2,463,472 3/1949 Bach 331-155 ELI LIEBERMAN, Primary Examiner L. ALLAHUT, Assistant Examiner U.S. Cl. X.R. 

